In recent years, with an upsurge of interest in environmental issues, attention has been focused on power storage systems that accumulate electric power generated by using renewable energy such as PV (Photo Voltaic). In the power storage systems, use of lithium ion secondary batteries that, unlike lead storage batteries, do not contain substances harmful for human bodies and the environment have been studied.
When a power storage system is configured by using lithium ion secondary batteries or the like (hereinafter, simply called “secondary batteries” generically), the power storage system generally adopts a configuration in which a secondary battery pack is formed by connecting a plurality of secondary batteries in series in order to obtain a desired output voltage, and a plurality of the secondary battery packs are connected in parallel in order to obtain a desired power storage capacity.
However, in the configuration in which a plurality of secondary battery packs (or secondary batteries) are connected in parallel, there is the fear that a cross current, in which a current flows between the secondary battery packs due to imbalance in the inter-terminal voltages of the respective secondary battery packs during operation, will occur and that the power storage system cannot be operated normally. Since among the secondary batteries, a difference occurs in deterioration state depending on production dates, temperature environments and the like, imbalance occurs in the inter-terminal voltages of the respective secondary batteries after charge and discharge, and due to imbalance, a cross current occurs among the secondary batteries, if there is a difference in the deterioration state even if the secondary batteries are of the same kind (specifications). This problem becomes more conspicuous as the number of secondary battery packs (or secondary batteries) that are connected in parallel becomes larger.
It is generally known that cross currents between secondary battery packs and between secondary batteries have an adverse effect on a power storage system, and many apparatuses that use a plurality of secondary batteries by connecting secondary batteries in parallel are prohibited from using a mixture of new and old secondary batteries.
However, in reality, a difference occurs in the rate of deterioration among the secondary batteries during operation even if all the secondary batteries are new. Further, there are many situations in which combined use of secondary batteries differing in deterioration state is desired, such as the case in which a power storage system is configured by using a small number of secondary batteries initially, and thereafter, secondary batteries are additionally provided, the case in which a power storage system is configured at low cost by using used secondary batteries, or the like.
From the background as above, the art of reducing user inconvenience by enabling the secondary batteries having different deterioration states to be used safely and freely in power storage systems has been desired.
As the art as above, for example, Patent Literature 1 describes a configuration that includes a plurality of secondary battery packs connectable in parallel with each other via switches, and eliminates imbalance between the inter-terminal voltages of the respective secondary battery packs at the time of discharge by controlling the respective switches provided at each of the secondary battery packs. In the art described in Patent Literature 1, the inter-terminal voltages of the respective secondary battery packs are respectively measured, and discharge is started from the secondary battery pack having the highest inter-terminal voltage. When the inter-terminal voltage drops due to discharge, and becomes substantially equal to the inter-terminal voltage of another secondary battery pack which has not been discharged until then, discharge of the other secondary battery pack is initiated. By performing control like this, the difference between the inter-terminal voltages of the respective secondary battery packs at the time of start of discharge can be reduced, and therefore, occurrence of a cross current can be prevented.
Incidentally, for example, in the secondary battery pack that is being discharged, the current that is supplied to a load is reduced when the other secondary battery pack that is connected in parallel with the secondary battery pack starts to be discharged, and therefore, the inter-terminal voltage rises. Meanwhile, in the secondary battery pack which is not being discharged, the inter-terminal voltage drops when discharge of the secondary battery pack that is not been discharged starts and thereby supplies a current to the load.
In the art described in the above described Patent Literature 1, the change in the inter-terminal voltages like this is not taken into consideration, and therefore, it cannot be said that the cross current which occurs between the secondary battery pack that starts to be discharged and the other secondary battery pack that is already being discharged and that is connected in parallel with the secondary battery pack when the secondary battery pack starts to be discharged is fully prevented.